In optical measurement technical fields or optical communication technical fields, optical waveguide elements having optical waveguides formed in a substrate having an electro-optic effect such as optical modulators or optical switches are widely used. Generally, these optical waveguide elements are housed in a case which is sealed and configured an optical waveguide element module.
As illustrated in FIG. 1, a relay substrate for electrically connecting input signals from the outside to a control electrode (a signal electrode and ground electrodes) in an optical waveguide element or a terminal substrate which is electrically connected to the output side of the control electrode in the optical waveguide element and is intended to terminate propagated electrical signals or lead out the propagated electrical signals to the outside of a case is housed in the case of an optical waveguide element module (refer to Patent Literature No. 1). In the present specification, relay substrates and terminal substrates will be collectively referred to as connecting substrates.
As illustrated in FIG. 1, for example, ground electrodes are formed at the input and output ends of the control electrode in the optical waveguide element so as to put a signal electrode therebetween. In addition, similar to in the control electrode, in a connecting substrate as well, ground lines are formed so as to put a signal line therebetween. In addition, the signal electrode and the ground electrodes at the input and output ends of the control electrode and the signal line and the ground lines in the connecting substrate are electrically connected (wire-bonded) to each other using wires such as gold wire.
In addition, the above-described optical waveguide element module (for example, an optical modulator) is widely used in optical communication technical field, but electrical cable(coaxial cable) for inputting electrical signals to the optical waveguide element module are generally designed to have an impedance of 50Ω, and thus it is desirable to design the signal electrode in the optical waveguide element and the signal line in the ground substrate which are directly or indirectly electrically connected to the electrical cable (the coaxial cable) to have an impedance of 50Ω in order to prevent electric characteristics from being deteriorated due to an impedance mismatch.
In addition, the optical waveguide element having an electro-optic effect is provided with a modulation portion having a length in a range of approximately several millimeters to several centimeters; however, in order to efficiently modulate optical waveguides having a width of several micrometers and a waveguide gap in a range of approximately several tens of micrometers to several hundreds of micrometers, the width of the signal electrode in the modulation portion becomes as extremely narrow as several micrometers to several tens of micrometers. Furthermore, since the distance from the input and output ends of the control electrode in the optical waveguide element to the modulation portion is short, the width of the signal electrode from the input and output ends of the control electrode to the modulation portion becomes, similar to the modulation portion, narrow. However, when the width of the signal electrode that is electrically connected to the connecting substrate is in a range of several micrometers to several tens of micrometers, wire-bonding (the diameters of wires are several tens of micrometer, and the number of bonded wires is two or three) is impossible, and thus a bonding area having, for example, a width of 100 μm or larger and a length of 100 μm or longer is provided in the input and output ends of the control electrode. Therefore, in the input and output ends of the control electrode (including the bonding areas), areas in which the widths of the signal electrode and the ground electrodes (GND) abruptly change exist. It is reported that the above-described abrupt change in the widths of the signal electrode and the ground electrodes (GND) acts as a cause for the generation of discontinuity of electrical connection and causes the electric characteristics of the optical waveguide element module to be deteriorated.
As illustrated in FIG. 2, in the related art (refer to Patent Literature No. 2), it is reported that discontinuity of electrical connection caused by the abrupt change in the widths of the signal electrode and the ground electrodes (GND) is suppressed by electrically connecting (wire-bonding) an area A in which the widths W1 of the ground electrodes on the optical waveguide element abruptly change and ends B of the ground lines on the connecting substrate using wires such as gold wire, and deterioration of the electric characteristics of the optical waveguide element module is prevented.
However, in the wire-bonding of the related art, bonding shapes (loop height, wire length, and the like) or bonding locations vary depending on operators' skills, and thus the suppression efficiency of the discontinuity of electrical connection is not stabilized, and consequently, there are cases in which desired electric characteristics cannot be obtained in optical waveguide element modules. Particularly, the lengths of wires connecting the ground electrodes and the ground lines become long, and thus the above-described disadvantage increases.
Although it is possible to stabilize the variation of the bonding shapes (roof height, wire length, and the like) or the bonding locations by means of the automation of wire-bonding using an exclusive device, it is still necessary to provide at least two long wires regardless of automatic or manual wire-bonding, the number of operation steps due to wire-bonding increases. In addition, there are cases in which long wires are disconnected due to vibration or impact, and thus there is a problem with deterioration of the electric characteristics of optical waveguide element modules due to the reliability of wire-bonding connection.